1. Field of the Invention
The present invention relates generally to an optical delay line and, more particularly, to a rotary optical delay line that can acquire data at high speed and can generate a long optical path difference.
2. Description of the Related Art
A pump-probe experiment is a very useful technology that can measure variations in various physical and chemical phenomena over time using very short electromagnetic pulses on the femtosecond scale. Such a pump-probe experiment is generally carried out using two short optical pulses. One of the short optical pulses is used as a pump optical pulse and is radiated to a target, which is desired to be observed, thus causes a desired reaction. Information about this reaction is acquired by a probe optical pulse. This probe optical pulse is a signal that is intentionally delayed by a predetermined period of time with respect to the pump optical pulse.
Accordingly, when information is collected from a reaction, which is desired to be observed, while changing the time delay value between the pump optical pulse and the probe optical pulse, variations in the time that it takes for the reaction to occur can be known. In the case where this technology is used, phenomena that occur on the picosecond scale, which is a very short period, can be observed with femtosecond-scale temporal resolution.
An optical delay line, which is used to generate the time delay between the pump optical pulse and the probe optical pulse, may be implemented in various ways. A typical optical delay line, which is chiefly used for experiments which require a long time delay, is configured such that a retroreflector is mounted to a motorized linear translation stage, and optical pulses are reflected while a retroreflector is linearly translated parallel to the traveling direction of the optical pulses. The path difference between the optical pulses is generated according to the location of the retroreflector, and the time delay value between the pump optical pulse and the probe optical pulse is determined by this path difference.
Furthermore, most experiments acquire signals having reduced noise by chopping one of the two optical pulses and using a Lock-In Amplifier (LIA) that is tuned to the chopping frequency. In the above-described measurement, the translation velocity of the linear translation stage is the most important factor that is used to determine the signal acquisition speed. Generally, when the motorized linear translation stage is used, a time delay of more than 100 picoseconds can be realized. In this case, the time ranging from several minutes to ten minutes is taken to acquire data.
FIG. 1 is a conceptual diagram illustrating a retroreflector, which is mounted to a conventional linear translation stage, and a time delay, which is attributable to the linear reciprocation of the retroreflector. In FIG. 1, the retroreflector reciprocates in a predetermined range in the same direction as the direction in which incident and reflected optical beams travel. A method of controlling the motion of the retroreflector may be implemented in various ways, in addition to the motorized linear translation stage.
In the method of FIG. 1, the time that it takes for the retroreflector to be returned to its original location is one period, and the delay time of an optical beam, which corresponds to a half of a total translation distance, which is obtained by translating the retroreflector for this period, is obtained. The above-described time delay method is disadvantageous in that it cannot be used for pump-probe experiments in which a repetition rate of more than several tens of hertz and a time delay of more than several hundreds of picoseconds are required.
In the case where the linear translation stage is used, the method has numerous limitations for applications that must rapidly process data because the data acquisition time is too long. In order to solve this problem, various types of technologies have been developed. An optical time delay line, based on a principle similar to the mechanical principle of a crank shaft, which converts the linear motion of an engine piston into rotational motion, has been commercialized and is used. First, a bar, which periodically reciprocates in a predetermined range, has been manufactured using an electrical signal, which varies periodically, a galvanometer and a mechanical device for converting rotational motion into linear motion.
Furthermore, in the case where a retroreflector is mounted to this bar, a device that can realize a time delay at a repetition rate of more than several tens of hertz may be manufactured. However, this device is disadvantageous in that it is difficult to obtain an actual time delay because the velocity of the reciprocating retroreflector varies according to a trigonometric function, rather than varying linearly over time. Furthermore, in the case where the repetition rate is increased, the maximum time delay value decreases, and thus a sufficient time delay value may not be obtained. On the contrary, in the case where a large time delay value is obtained, the repetition rate decreases, and thus the data acquisition speed becomes slow.